1. Field of the Invention
The present invention relates to a demodulator for radio communication, and, more particularly, to a demodulator capable of demodulating any of a full carrier SSB (Single Side Band) signal, a reduced carrier SSB signal, an RZSSB signal, and an FM signal, and a carrier signal reproducing circuit used in the demodulator.
2. Description of the Related Art
In mobile communication or the like, an RZSSB (Real Zero Single Side Band) system has been proposed as a radio communication system with an excellent frequency usage and excellent fading characteristics.
A conventional demodulator for demodulating an RZSSB signal comprises: an FM demodulator 71, which includes an amplitude limiter 73, a frequency detector 74 and an integrator 75; and a linearizer 72, as shown in FIG. 4. The linearizer 72 comprises a Hilbert transform circuit 81, a delay circuit 82, mixers 83 and 84, a cuber 85, amplifiers 86 to 88, and an adder 89, as shown in FIG. 5, for example.
The FM demodulator 71 demodulates an incoming RZSSB signal, and the linearizer 72 eliminates the high-order distortion of the demodulated signal included in a demodulated signal.
The conventional RZSSB demodulator requires the linearizer 72 to eliminate the high-order distortions, as shown in FIG. 4, and this linearizer 72 should include the Hilbert transform circuit as shown in FIG. 5.
The Hilbert transform circuit demands the complicated circuit design and is difficult to manufacture. Further, while the characteristics of the Hilbert transform circuit greatly affect the quality of the demodulated signal, it is difficult to design and manufacture a Hilbert transform circuit having excellent characteristics.
Although the distortion of third order or greater can be reduced to a negligible level without using the linearizer 72 if the modulation index of the RZSSB signal is set small, the demodulation efficiency is impaired.
As mentioned above, the conventional RZSSB demodulator can eliminate the distortions of the RZSSB signal using the linearizer 72. When an FM signal is to be received, however, the demodulated signal contains no distortion so that the linearizer 72 produces distortions in the demodulated signal by contraries. Therefore, the conventional RZSSB demodulator cannot demodulate an FM signal, so that the demodulation of both the SSB signal and FM signal requires separate demodulators designed specifically for those signals.